Various types of analytical tests related to patient diagnosis and therapy can be performed by analysis of a liquid sample taken from a patient's infections, bodily fluids or abscesses. These assays are typically conducted with automated clinical analyzers onto which tubes or vials containing patient samples have been loaded. The analyzer extracts a pre-determined volume of liquid sample, typically in the range of 1–3 mL, from the vial using an appropriate probe and combines the sample with various reagents in special reaction cuvettes. Analytical measurements are often performed using a beam of interrogating radiation interacting with the sample-reagent combination, for example turbidimetric, fluorometric, absorption readings or the like. The measurements allow determination of end-point or reaction rate values from which an amount of analyte related to the health of the patient may be determined.
An important aspect of maintaining analytical accuracy in such analyzers is the ability to precisely extract the pre-determined volume of liquid sample from the vial. At the same time, in order to minimize cross contamination between samples and reagents and facilitating cleaning the tip the probe, it is desired to minimize contact between the probe and the liquid. Therefore the probe is introduced into the liquid container and preferably maintained a short distance below the surface of the liquid. Liquid aspiration is then accomplished by either aspirating the pre-determined volume while the probe is stationary (for very small volumes) or moving the probe further into the probe during aspiration (for larger volumes).
Various methods have been implemented to locate the uppermost level of liquid in the container, frequently employing capacitive level sensors. Such sensors are based on the fact that any charged conductor, like a probe in air, exhibits a finite electrical capacitance relative to a ground and this capacitance will change if the probe is placed in contact with a medium other than air. In particular, when the probe contacts liquid, its dielectric constant increases above that in air and the greater surface area of the liquid results in an increased probe capacitance. These capacitance changes can be very small so that sensitive detection devices are required and these must be free of false signals arising from electrical disturbances, contaminations, bubbles and the like.
U.S. Pat. No. 6,164,132 discloses a capacitive liquid level sensor having a capacitive sensor array superposed on each side of a dielectric substrate, wherein the sensor signal detection electronics are located immediately adjacent each capacitive sensor. These provisions result in high sensitivity of detection of submergence in the liquid, as well as essentially eliminating parasitic electric fields. The preferred capacitive sensors are interdigitated capacitors, and the preferred sensor signal detection circuit is an RC bridge and a comparator. The sensitivity of the capacitive liquid level sensor allows a reference capacitive sensor to be obviated, so that there are no false indications of liquid level due to any film of the liquid clinging to an exposed portion of the capacitive liquid level sensor.
U.S. Pat. No. 5,493,922 discloses a liquid level sensor control circuit for controlling the position of a sampling probe relative to a liquid in a container. The apparatus includes a sampling probe, an oscillator circuit coupled to the sampling probe for producing a first output signal having a constant frequency, a comparator coupled to the oscillator circuit for comparing the amplitude of the first output signal to a first reference amplitude and for producing a change signal when the amplitude of the first output signal changes with respect to the reference amplitude, and a controller responsive to the change signal for controlling the position of the sampling probe with respect to the surface of the liquid.
U.S. Pat. No. 5,437,184 discloses a capacitive liquid level sensor having phase detecting circuitry in a capacitive sensor array. An X-OR circuit generates a first logic level signal when a difference in the phase of two signals from any two adjacent output plates indicates that a phase difference is present. A second logic signal is generated if no phase difference is detected. The signals are perfectly in phase when any two adjacent output plates are either submerged in fluid or both disposed in air.
U.S. Pat. No. 5,365,783 discloses a computer controlled pipette probe for aspirating or dispensing liquid in the vessel. The charge developed via the capacitance on the probe is coupled to a capacitive sensor circuit which provides a peak detector with an amplified signal representing the peak capacitance between the probe and the liquid. This amplified signal is detected by a peak-capacitance discrimination circuit, the output of which is monitored by the computer for determining the precise position of the probe with respect to the liquid surface level.
U.S. Pat. No. 5,083,470 minimizes false level sensing problems associated with capacitive liquid level sensors by isolating the probe from the connecting tubing by the use of an element exhibiting inductive reactance.
A number of other related U.S. Patents include: U.S. Pat. No. 6,101,873, having a plurality of electrodes positioned vertically from the liquid surface and a level detection circuit for detecting level of the liquid by measuring variations of the capacitance measured between the electrodes; U.S. Pat. No. 5,600,997, wherein a capacitive probe is located at a predetermined desired fluid detection level and when the fluid level recedes, the capacitance of the system changes; U.S. Pat. No. 5,451,940, having a measuring capacitance and at least one reference capacitor; U.S. Pat. No. 5,051,92, using two capacitors to produce a signal proportional to the level of the liquid in the tank and to the composition or dielectric constant of the liquid; and, U.S. Pat. No. 4,908,783, sensing the level of fuel in aircraft fuel tanks using a plurality of capacitive sensors which provide an output capacitance that is a function of the fraction of the sensor wetted by the fuel.
Accordingly, from a study of the different approaches taken in the prior art to provide very sensitive liquid level detection devices, there is a need for an improved approach to ascertain when false signals are generated within capacitive liquid level sensors. In particular, there is a need for a method to confirm that a change in capacitance within a liquid level sensor arise from true physical contact between a probe and a liquid, and that such a change in capacitance is not caused by other factors capable of generating false signals.